- The last stage of barrier formation occurs within the extracellular spaces of the stratum corneum (SC), a sequence involving a family of lipid catabolic enzymes. Lamellar bodies (LB) deliver not only a mixture of nonpolar lipid, but also a family of lipid hydrolases to the SC interstices. Moreover, degradation of the plasma membrane makes an additional pool of lipid precursor, particularly sphingomyelin (SM), available for barrier formation. These enzymes are presumed to regulate the transformation of polar lipids (glucosylceramides, GlucCer; SM; phospholipids (PL); cholesterol sulfate, CS) to the three-component mixture of nonpolar lipids; i.e., ceramides (CER), free fatty acids (FFA), and cholesterol (CHOL), which form the lamellar unit structures required for barrier function in a terrestrial environment. Although considerable information is available about GlucCer-to-Cer processing, little is known about the regulation and requirement of PL-to-FFA, sphingomyelin SM-to-CER, and CS-to-CHOL processing. Moreover, the role of the SC pH gradient as an in situ regulator of lipid processing, and the sequential orchestration of lipid processing remains unexplored. Despite recent evidence that retinoid receptors regulate lipid processing transcriptionally, the target processing enzyme(s) is (are) not known. In this grant proposal; the investigators will assess: a) changes in lipid processing enzyme activity, content, mRNA, and localization in relation to barrier requirements; b) requirements for key processing steps in transgenic knockout mutant, and inhibitor-based models; c) the in situ regulation and orchestration of lipid processing; e) whether one or more lipid processing enzymes are regulated transcriptionally by retinoid receptors.